The term "nuclear meltdown" has become synonymous with worst-case scenarios. This is true whether you're talking about your boss' figurative meltdown in the office or the very real concerns surrounding such events as the 2011 Fukushima Daiichi nuclear power plant disaster.

Indeed, as nuclear power plants can't produce a Hiroshima-style nuclear detonation, a meltdown is about as bad as it gets. Numerous nuclear meltdowns have occurred throughout humanity's atomic age, though thankfully only four large-scale events have ever occurred at civilian plants. The first took place at the Swiss Lucens reactor in 1969. The Three Mile Island accident occurred a decade later, followed by Russia's Chernobyl disaster in 1986 and the Fukushima Daiichi mess in 2011.

The International Atomic Energy Agency (IAEA) ranks nuclear events on a scale of zero to seven, ranging from a mere deviation with no safety significance (level 0) to a major accident (level 7) such as Chernobyl in which widespread health and environmental damage occurs.

Interestingly enough, neither the IAEA nor the U.S. Nuclear Regulatory Commission officially recognizes the term "nuclear meltdown," but the words continue to inspire fear. In this article, we'll break down how a nuclear reactor works and how a meltdown can occur.

Don't worry too much over complex equations, because the whole situation ultimately comes down to heat. Properly controlled heat inside a reactor helps generate power. Out-of-control heat, on the other hand, can cause the reactor itself to melt and contaminate the surrounding environment with dangerous radiation.

Now head to the next page to learn what happens inside a healthy reactor.